Implements securitygroup extension for nuage plugin

Implements securitygroup extension for nuage plugin

In Nuage VSP, the scope of vport-tag(Neutron securitygroup equivalent) is either
per router or per subnet whereas securitygroup scope in Neutron is per tenant.
Because of this, the mapping between neutron and VSP resource always happens at
the port create or update time, such that port's router/subnet is known and
thus sg attachment point in VSP is known.
On port-update --security-group sg1, if this is the first port getting attached
to that security-group, corresponding vport-tag(for sg) and rules( for sg rules)
are created on VSP. Subsequent port-update for the same sg1 will just update the
port to vport-tag binding. When security-group and security-group-rules are
deleted on neutron, corresponding vport-tag and rules are deleted on the VSP.

Implements: blueprint securitygroup-ext-for-nuage-plugin

Change-Id: If6f7f16ad727ae9394008fb81b56c785c2404c8f